A superlattice (SL) is a structure consisting of closely spaced quantum wells, such that the localized discrete energy levels in the quantum wells become delocalized minibands across the entire structure, in both the conduction and valence bands. These minibands thus form an effective bandgap (Ege) for the superlattice considered as a whole.
Type II superlattice systems are attractive to groups with both research and commercial interests because, for example, (1) the minibands of the superlattice can exhibit a lower effective bandgap than the bandgap of either constituting layer, (2) the electrons and holes are spatially separated, and (3) electrons can easily tunnel from the conduction band of one layer to the valence band of the other layer (i.e. Zener tunneling) because the energy of the conduction band of the former layer is less than the energy band of the later layer. A type II InAs/GaSb superlattice system is a particularly attractive quantum system because of its flexibility in designing the interband transitions over a wide range of wavelengths, for example, from 2 to 32 μm. Furthermore, a reduced Auger recombination rate results in small dark current at high temperatures.
However, when operating at low temperatures, for example, 77 K, the doping level is a critical parameter in determining the electrical and optical quality of a narrow gap device. The electron affinity of GaSb is approximately 4.05 eV, and the electron affinity of InAs is approximately 4.90 eV. This difference creates a broken-gap type II configuration; that is, the conduction band of the InAs stays below the valence band of the GaSb. When successive layers of GaSb and InAs are grown to form a superlattice, the electrons are attracted to the InAs layers and the holes are attracted to the GaSb layers.
Dark current mechanisms for the net transport of electrons and holes across the depletion region can be classified into two categories: (1) inherent or fundamental mechanisms, which depend only on the material properties and device design, and (2) defect-related mechanisms, which require a defect as an intermediate state. When operating at 77 K, the performance of an InAs/GaSb superlattice is limited by defect-assisted tunneling.